Abstract
This report aims to find the influence of work materials on drill breakage when machining hardened sintered steels, and to clarify the mechanisms of the drill breakage. The work materials having different hardness obtained by the different heat treatment were prepared, and drilling, and turning tests were carried out. The main results obtained are as follows : Fracture of the drill occurs when drilling materials of high hardness. During drilling the high hardness sintered steels, cutting temperature becomes high, and so that thermal expansion caused a high friction between the drill and surface of the hole drilled. In the effect the drill breaks down, when the friction force exceeds the drill's twisting strength. However, in drilling the low hardness sintered steels, the cutting temperature becomes low, and the thermal expansion is also small, furthermore yield strength of their steels is small, so that the friction force does not become large, and consequently the drill does not break down.
